[TASK] Rename "getArguments" to "getVariables" on AbstractAssetViewHelper

Helps avoid confusion with actual arguments on ViewHelper; the "variables" here are actually overlaid using TypoScript configuration.
[FEATURE] Rendering using Fluid for Fluid-enabled Assets

Requirements: "path" must be used, "fluid" must be enabled. And very most likely, "arguments" must be added to transfer some arguments.